Exterior surface of a heat sink



FIG. 1 is a perspective view of an exterior surface of a heat sink embodying my new design;

FIG. 2 is an end view thereof, the opposite end being a mirror image of this view;

FIG. 3 is a side elevation view thereof which is identical from both sides;

FIG. 4 is a top view thereof; and,

FIG. 5 is a bottom view thereof.

The broken line showing in FIGS. 1, 2 and 5 is included for the purpose of illustrating environmental elements only and forms no part of the claimed design. 

The ornamental design for an exterior surface of a heat sink, as shown and described. 